Radio monitoring system



Oct. 3l, 1944. w. VAN B. ROBERTS RADIO MONITORING- SYSTEM Filed March 14, 1942 SKT Patented Oct. 3l, 1944 RADIO MoNrroRlNo SYSTEM Walter van B. Roberts, Princeton, N. J.,lassign'or to Radio Corporation of America, a` corporation of Delaware Application March 14, 1942, Serial No. `434,688 commis. (o1. 25o-20) My present invention relates to radio signalling systems, and more particularly to a radio receiving system `including monitor devices.

`To be useful a television system should include means for lcalling attention to unscheduled television transmissions. This is true because a large part of the interest in television may, in the future, reside in telecasting of news events which by their unexpected nature are'likelyto occur at times whenno program is scheduled. To. some extent the same is true o f sound broadcasting. However, in the latter case the broadcasting transmitter is usually on the air, but the receiver `may not be turned on or tuned to the station that has some announcement of importance to make. In the case of facsimile transmission, the transmitted information is not lost if the receiver is arranged to run all the time. However, in the case of facsimilevtransmission, the material may be unnoticed at the facsimile receiver for some time.` i

In accordancewith the present invention a combination facsimile receiver and alarm system is provided which is adapted to monitor all the aforesaid services so that important material may be `called to the attention of the householder without delay. p l

An important object of the present invention is to provide a receiving system wherein the detecting network is continually operative, there being` provided a relay device forv energizing the rest f the receiver solely while a carrier is being received. Y

Another important object of the invention is to provide in a signalling system, a method of transmitting on a carrier one type of modulation which is representative of facsimile signals, and another type of modulation which is representative of `an alarm signal, and/in the case of the latter type of modulation there being utilized a low per centage amplitude modulation.

Another object of the invention is to provide a receiving system for frequency modulated carrier Wave energy wherein nat frequency response transformers are utilized prior to ther demodulatem of a radioreceiver, the magnetic coupling being `purely motional and transmitting special modulation tones. f

The novel features which Ibelieve to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both `its organization and method of operation` will best be understood by reference to the following description `taken in connection with the drawing in which Ihavein-Y dicated diagrammatically, a circuit organization whereby my invention may be carried into ef` fect. i

Referring to the accompanying drawing, there is showna receivingsystemthat canbeusedin connection with the present invention., As stated heretofore, the receiver is a combination` facsimile receiver and alarm system wherein important matter maybe called to the attention of the householder ,atv such times when it `is desired lto transmit unscheduled or unexpected material. For the-sake of simplicity of illustration, certain networks of the receiving' system are shown in schematic form..` Let itbe assumed thatthe facsimile signals are vapplied to the carrier at the transmitter by frequency modulation. In other words, the carrier at the transmitterV has a substantially constant amplitude, and the facsimile signals are utilized to vary the carrier frequency in a manner well known to those skilledin the art of frequency modulation. g

Merely by way of example, the carrier maybe in the ultra-short wave range, such as megacycles (mc.) Of course, the invention is in no way) limited to that carrier frequency value, since frequencies below that band may be employed. The alarm signals areto beunderstood as vap-4 plied in the `form of amplitude'modulation to the carrier. In other words, the carrier amplitude will b e varied at the particular modulationrtone, or tones, representative of different types of alarm signals whichmay be utilized. Those skilled viny the art of radiocommunication are fully aware of the particular manner in modulating a carrier with one type of modulation such'as frequency modulation, and at the same time modulating the same carrier 'with another type of modulation such as amplitude modulation.

At the receiver the radiated modulated carrier energy is collected at an antenna A, and the latter may be of any desired type. VThere are shown at least two stages of radio frequency amplication, and each stage being provided with tuned input and output circuits. Each of the transformers T, T1 and T2 has its primary and secondary circuits each tuned to the incoming carrier frequency. In

detector, or demodulator, which is represented as of the diode type. The detector load resistor I de- .velops amplitude modulation signal components which are applied to the carrier in they form of alarm signals. An AVC (automatic volume control) circuit is shown connected to the cascaded radio frequency amplifiers so as to maintain a substantially constant carrier amplitude at the input circuit of the demodulator.

The tubes of the cascaded radio frequency amplifiers and the detector diode are kept energized continually from a preferably special power source. rIt will be understood that the filaments, or heater elements, of these various tubes are connected to said special power supply source. The plate circuits of the radio frequency amplifiers are shown connected to a plate energizing connection 2. The symbol +B designates such a positive potential source. v The eelctromagnetic relay 3 is included in circuit with the plate supply voltage connections to the radio frequency amplifiers. With modern low current tubes used in the stages up to, and including, the detector much power is not required, and the tubes have long life. When no signals are coming in there is little, or no, AVC voltage developed across resistor I. The plate current flowing through connections 2 and the winding of relay 3 are normal, and hold the pivoted contact, or armature, element 4Vaway from the contact point 5. A spring is provided to pull armature 4 away from the electromagnetic relay 3, thereby closing switch 4-5,'when the AVC action causes the plate current flow through the relay winding to decrease to a low value.

The switch 4-5 is shown inserted in series with the primary winding of. a power supply transformer` 6. The secondary winding of the transformer vis connected to the usual power rectifier 1. The filtered rectified power supply voltage is fed to the plates of all tubes which follow the demodulator of the system. An auxiliary secondary winding is provided on the power supply transformer 6, and is adapted to connect to the heater elements, orlaments, of all of the tubes following the demodulator. This is purely illustrative. It will be understood that switch 4--5 could bein the secondary circuit of transformer 6. Further, the power for the demodulator vand prior stages could come from the source supplying transformer 6. l A

When a modulated, carrier is collected by antenna A, AVC bias is developed across resistor] with the result that the current flowing through the winding of relay 3 decreases. yThis results in closure of switch 4-4, and allthe tubes following the diode detector are immediately ener; gized. There are two main channels which follow the diodedetector. One of these is a facsimile receiver, while the other is a group of selectively operated alarm bells, or other indicator devices. In the present example, facsimile signals are derived from the frequency modulation of the carrier, While the alarm signals are derived from the amplitude modulation of the .same carrier. Of course, these two modulations might be interchanged. i

Considering first the frequency modulation (FM) portion of the system, it will be understood that a limiter network, designated by the numeral 8, is employed. This limiter functions to wipe out any amplitude modulation (AM) effectsof the modulated carrier wave prior to impression of the modulated carrier lenergy on the discrimi. nator-rectier 9. Adjacent to the rectangle desthis way suicient selectivity is provided for the 1 ignated AM Limiter there is shown in purely pictorial manner the limiter input-output characteristics. Those skilled in the art are fully acquainted with this characteristic, as well as with the usual S-shaped characteristic of the diseliminator-rectifier 9. There will be fed to the network 9 modulated carrier energy which is variable solely in frequency. The limiter network 8 will remove any of the amplitude modulation tones which are representative of the alarm signals. The output of the network 9 will be the facsimile modulation signals. The numeral I B denotes a facsimile recorder devicey of any well knownand desired construction. The manner of constructing -a facsimile recorder is too Well known in the art to require description. It is to be understood that any well known type of re corder may be used.

Considering now the second channel of the receiving system, it will beunderstood that the modulation voltage developed across the diode load resistor I. Which is by-passed by the carrier rby-pass condenser I', will be representative solely of the AM signals which Were applied to the carrier at the transmitter. By way of example, there is shown herein the case wherein a modulation tone A, and a different modulation tone B, haveA been applied to the carrier. Of course, a single modulation tone may be utilized, or more than two tones may be employed. A coupling condenser II, which passes the modulation tone frequencies, connects the anode end of resistor I to thefinpu't terminals of the modulation tone ampliers I2 and I2. There will be described in detail the construction of the circuits between the amplifier l2 and the indicator device. It will be understood that the same construction is employed in the case of the circuits following the modulation amplier I2.

"Since each of amplifiers I2 and I2 is fed with combined modulation tones, there must be provided a frequency selective device to separate for each indicator its respective modulation tone. There is shown employed a vibratory reed I3 having one end xedly secured to a support. The free end of the reed is provided with a magnet I4. One pole of the magnet is located adjacent a driving coil I5. One end of the driving coil is grounded while its opposite end is coupled by condenser I6 to the upper end of the plate Voltage feed coil I'I ofamplifier I2. As explained previously, the plate feed coill I 'l is included in the power supply circuit followingpower rectifier 7.

The opposite pole of magnet I4 is located adjacent a coil IB, one end of which is connected to the anode of a diode rectifier I9. The opposite end of coil I8 is connected to the cathode of diode I8 through the winding of electromagnet 20. The winding of electromagnet 20 is bypassed for the modulationv tone frequency. 'Ihe electromagnet 20 is arranged to actuate a normally-open switch device 2|. The switch device 2| is arranged in circuit with any desired type of indicatordevice, such as a bell, lamp, or buzzer.

To explainithe operation of the alarm portion of the system, it will be understood that the amplified modulation tones in each case drive a mechanically tuned vibrating reed I3, and that each reed carries a permanent magnet upon which the modulation tone current acts. When the modulation tone frequency is equal to that of its respective vibrator, the motion of the latter induces a current in its respective coil I8. The induced current is rectified by diode I 9 thereby causing switch 2l to close. Such vibrating VIl" Ireed devices vare cheap and, easy to make; they Let us assume that the vibrator reed associated with amplifier I'2 is tuned to 100 cycles, and that when the transmitting station has been informed .that an unscheduled television transmission is taking place an amplitude modulation of 100 cycles, is impressed on the carrier. switch 2| to be closed. `As a result a bell, or a buzzer, is energized for so long as the modulation tone of 100 cycles is applied to the carrier. Of course, the modulation tone may be continuous, or repeated at intervals at the discretion of. the transmitter. At the same time the facsimile. material applied to the carrier is interrupted to insert a description of what the alarm bell was rung for. If the transmitter carrier had not been on when the alarm was to be given the procedure would have been the same, except that a few seconds would have to be allowed for tube I2 to heat up.

In case the householder does not also own a television receiver, he would probably not care for the television service described above. He might, however, like to be informed that something important was being announced over the sound radio stations. In that case he would have his facsimile receiver equipped with a relay tuned to say 90 cycles, so that his bell would ring only on 90 cycle modulation. The latter would be used by the transmitter when, for example, an address of national importance to all the people was to be put on the air at an unscheduled time.

As stated heretofore, the present receiving system is notlimited to a single service. Tube l2', which is in parallel with the input terminals of amplifier I2, will lead to a similar alarm circuit except for the modulation tone frequency to be employed. The bells, or other indicators, associated with the parallel alarm circuits should preferably be aurally distinguishable. In a simplified embodiment the outputs of tubes l2 and could actuate tuned vibrator reeds which would strike directly on different bells thus providing differentiation both by the tones of the bells and by the frequency of the clapper blows. It is, furthermore, possible to codevthe informa tion given by the operation of the relays. For example, one relay circuit may be arranged to strike a rich-toned bell on the hour to act like a clock. This same bell, if rung continuously, would constitute a call to come to the facsimile recorder, and read what it says is going on. As another example, a code of ringing would give notice that the recorder hlad just `printed a weather report, or a war news bulletin, indicating by the ringing code which it is. A

It is to be clearly understood that the nature of the modulated carrier wave is purely illustra tive. For example, there could be employed a carrier which is amplitude modulated by a subcarrier. while the sub-carrier is frequently modulated by the facsimile modulation signals. Such type of modulated wave has been used in facsimile transmission. In such case, the main carrier would be amplitude modulated by the modulation tones representative of the alarm signals. In this type of modulated wave, it is only necessary to subject the main carrier to amplitude modulation to secure the frequency modulated subcarrier, and the latter would then This causes due to round-peaked response curves.

be subjected to the frequency modulation detection as. described in this application.

- Each of transformers T. Ti and T2 desirably has a' fiat frequency response characteristic so that undesired amplitude modulation components will not be developed. Such components are produced on the frequency modulated carrier departing from the scope of my invention, as set forth inthe appended claims.

What I claim is: 1. In a radio signalling system, a receiver comprising a modulated carrier wave amplifier followed by a demodulator, means for continuously energizing said amplifier and demodulator, means for applying to said amplifier modulated carrier waves whose modulation includes modulation signals of two different types, a network coupled to the amplifier output for utilizing modulation of one type, a second network coupled to the demodulator output for utilizing the modulation of the second type, a power supply circuit for energizing both said networks, and means, responsive to carrier-derived energy, for rendering saidfpower supply circuit operative to energize both said networks.

2. In a radio signalling system, a receiver comprising a modulated carrier wave amplifier followed by a demodulator, means for applying to said amplifier modulated carrier waves whose modulation includes modulation signals of two different types, a network for utilizing modulation of one type, a second network for utilizing the modulation of the second type, a power supply circuit for both said networks, and means, responsive to carrier-derived voltage, for rendering said power supply circuit operative to energize both said networks, one of said modulations being applied to the carrier in the form of frequency modulation, and the other type of modulation being applied to the carrier in the form of amplitude modulation.

3. In a radio signalling system, a receiver comprising a modulated carrier wave amplifier followed by a demodulator, means adapted to continuously energize said amplifier and demodulator, means for applying to said amplifier modulated carrier waves whose modulation includes modulation signals of two different types, a network for utilizing modulation of one type, a second network for utilizing the modulation of the second type, a power supply circuit for both said networks, and means, `responsive to carrier-derived energy, for rendering said power supply circuit operative to energize both said networks, said second network including an indicator device, and a vibratory magnetic device selectively coupling said indicator to the output of said de modulator.

4. In a radio signalling system, a receiver comprising a modulated carrier wave amplifier followed by a demodulator, means for applying to said amplifier frequency modulated carrier waves which are amplitude modulated by a tone frequency, a network coupled to the amplifier for utilizing the frequency modulation, a second'network coupled to the demodulator for utilizing the tone voltage output of thelatter, a power supply circuit for both said networks, and means, re-

sponsive to carrier-derived current, for rendering said power supply circuit operative to energize 'l both said networks.

5. In combination, a modulated carrier wave transmission network including a demodulator, said network being conditioned for wave reception, means for applying to said network fre` quency modulated carrier waves whose carrier is amplitude modulated by at least one modulation frequency, limiter means coupled to said network in parallel with said demodulator and at a point prior to the demodulator input for removing the l amplitude modulation from the modulated wave, means detecting the limited wave energy, means utilizing the detected energy, means coupled to the demodulator output for providing voltage of said one modulation frequency, means for utilizing said voltage and additional means, responsive solely to reception of waves by said network, for rendering said limiter'means, detecting means and said demodulator output means operative.

6. In` combination, a modulated carrier 'wave transmission network, including a demodulator,

- means for applying to said network frequency modulated carrier waves whose carrier is amplitude modulated by at least one modulation frequency, limiter means coupled to said network Iin parallel with said demodulator and at a point prior to the demodulator input for removing the amplitude modulation from the modulated wave,

means detecting the limited wave energy, means utilizing 'the detected energy, means coupled to the demodulator output for providing voltage of said one modulation frequency and means for utilizing said Voltage, said'network and demodu- -lator being in a state of continuous energization, 

